1. Field of the Invention
This invention relates generally to an acceleration detecting device which detects acceleration or deceleration and more particularly to an acceleration detecting device with a beam supported at both ends.
2. Description of the Prior Art
The acceleration detecting device shown in FIG. 17 is well known in the prior art. This detecting device comprises a beam 1 whose one end is supported by a plinth 3 and whose other end is free, with strain gauges 2 provided on the beam 1.
When an acceleration is applied to this detecting device, the free end of the beam 1 is displaced, and a bend is generated on the beam 1. Then the bend generated on the beam 1 is converted into an electric signal by the strain gauges 2.
Meanwhile, the bend generated on the beam 1 is directly proportional to the acceleration. Accordingly, the electric signal converted by the strain gauges 2 becomes directly proporational to the acceleration.
Such a detecting device is suggested in many applications, for example Japanese patent publication No. 139758/1986 published on June 27, 1986. The Japanese patent publication No. 139758/1986 shows a semiconductor detecting device with a silicon beam and strain gauges 2 made of piezo-electric resistors.
In the above mentioned detecting device, the beam 1 has to be bent greatly by a small acceleration in order to detect a small acceleration.
It is possible to amplify the electric signal converted by the strain gauges 2 in order to detect an acceleration with a smmaller level. However, it is preferable not to amplify the electric signal from the detecting device, because not only the necessary electric signal but also noises are amplified at the same time.
Weakening the mechanical strength of the beam 1, for example by thinning the beam 1 or by increasing the mass of the free end of the beam 1 by fixing a weight to the free end etc., is required in order to bend the beam 1 greatly by an acceleration with a smaller level.
However, if the mechanical strength of the beam 1 is weakened, an accurate detection could not always be obtained after applying an excessive acceleration. Because, the detecting parameters might be changed by breakage or by plastic deformation of the beam 1 etc.